1. Field of Invention
The invention relates to a circuit arrangement for supplying an LED array.
2. Description of Related Art
A circuit arrangement for supplying an LED array may include input terminals for connection to a voltage supply source, output terminals for connection to the LED array, and a DC—DC-converter coupled between the input terminals and the output terminals and equipped with an inductive element L, a unidirectional element, a switching element coupled to the inductive element and the unidirectional element, and a control circuit coupled to a control electrode of the switching element for generating a high frequency control signal for rendering the switching element conductive and non-conductive at a high frequency to thereby operate the DC—DC-converter in the critical discontinuous mode and equipped with means I for controlling the current through the output terminals at a predetermined value.
Operation in the critical discontinuous mode means that the current through the inductive element L equals zero at the beginning and at the end of each period of the control signal, while it differs from zero during each period of the control signal. This mode of operation ensures a high efficiency since power losses in the unidirectional element are prevented to a large extent. In the known converter the means I for controlling the current through the output terminals consist of a current control loop equipped with feedback. The actual value of the current is measured and compared with a desired value by means of a comparator that generates an error signal that in turn adjusts the control signal in such a way that the actual value of the current through the output terminals substantially equals the desired value. An advantage of such a control loop is that it allows a very accurate control of the value of the current. Disadvantages, however, are that the control loop is expensive since it comprises many components and that the control loop is comparatively slow. Furthermore, in case the actual value of the current is measured by measuring the voltage across an ohmic resistor that is placed in series with the output terminals, the control loop also causes a substantial power dissipation.